In physics class, the girl to boy ratio is $10$ to $5$. If there are a total of $30$ students, how many girls are there?
Explanation: A ratio of $10$ girls to $5$ boys means that a set of $15$ students will have $10$ girls and $5$ boys. A class of $30$ students has $2$ sets of $15$ students. Because we know that there are $10$ girls in each set of $15$ students, the class must have $2$ groups of $10$ girls each. There is a total of $20$ girls in physics class.